(1) Field of the Invention
The present invention relates to a loop transmission system and to a method of controlling the loop-back condition thereof, in which loop-back paths from one of two duplicate transmission lines to the other duplicate transmission line are formed in a short time if the duplicate transmission lines become faulty.
(2) Description of the Prior Art
As is illustrated in FIG. 1, in a conventional loop transmission system, a plurality of node stations ND1, ND2, ND3, etc. and a supervisory station SV are mutually connected by duplicate transmission lines TL1 and TL2, and signals are transmitted on the duplicate transmission lines TL1 and TL2 and through the supervisory station SV in opposite directions. In a normal condition, one of the duplicate transmission lines, for example, TL1, is used as an active route, i.e., a "0" route, and the other duplicate transmission line, for example, TL2, is used as a standby route, i.e., a "1" route. If only the duplicate transmission line TL1 becomes faulty at any point thereon, the supervisory station SV detects the fault and immediately switches the duplicate transmission lines so that the standby transmission line TL2 is used as an active route. Therefore, there can be normal communication between each node station. However, if two routes become faulty at the same time, for example, if both of the duplicate transmission lines TL1 and TL2 are cut at the same time, communication on both the active transmission line and the standby transmission line is interrupted. When such a fault is detected, the supervisory station SV connects the input side and the output side of the transmission lines in the node stations on both sides of the cut portion of the transmission lines. For this purpose, there is provided a connection for loop-back, i.e., for returning signals transmitted to each of the node stations. The connection includes a switch which is turned on by the output signal of a control unit which decodes the command sent from the supervisory station SV. To communicate by the transmission path constructed by using the transmission lines in both directions is called "loop-back". If a fault occurs on one or both of the transmission lines, the supervisory station SV can detect the fault from the disturbance of the clock pulses, the disorder of the received frames, and so forth. However, it is impossible to immediately judge where the fault has occurred between two node stations because it generally occurs in a remote place. In order to determine where the fault has occurred, the node stations are sequentially checked, beginning with the node station ND1, which is near the supervisory station. That is, the node station ND1 is, at first, placed in a loop-back condition. If a signal sent from the supervisory station SV to the node station ND1 returns normally to the supervisory station SV, it is determined that the transmission route from the supervisory station SV to the node station ND1 is in a normal condition. Therefore, the loop-back connection of the node station ND1 is released. Then the node station ND2 is placed in a loop-back condition and checked whether or not a signal sent from the supervisory station SV returns normally thereto through the node station ND2. Next, the other node stations are sequentially placed in a loop-back condition and checked whether or not a signal sent from the supervisory station SV returns normally thereto through the node stations. If the signal does not return normally, it is temporarily determined that a fault has occurred between the node station through which the signal sent from the supervisory station SV does not return normally and another node station preceding that node station. Then the supervisory station SV checks every node station in the opposite direction in a similar manner and detects the faulty portion of each of the transmission lines. If it is detected that the same portions of the transmission lines are faulty, the node stations on both sides of the faulty portions are placed in a loop-back condition and communication is resumed by using the transmission lines of both routes and the loop-back paths.
However, in the above-mentioned conventional loop transmission system, it is necessary to perform complex operations for detecting a faulty portion and it is necessary to interrupt communication for a long period of time, especially when the number of node stations is large.